Flat mirrors mounted on resonant torsional galvanometers, as illustrated in U.S. Pat. No. 3,978,281, are frequently employed as the scanning means in TV compatible radiation scanning systems. Resonant mechanical scanners are highly reliable with an almost indefinite maintenance-free service and shelf life, and are also relatively low in cost and power consumption. Resonant mechanical scanners in general, however, have some inherent limitations which degrade the efficiency of radiation scanning systems.
Firstly, scanning is accomplished at a sinusoidal rate, resulting in non-linear scanning near the scan angle limits which reduces the overall scan efficiency of the system. In passive scanning systems, moreover, the sinusoidal scan rate restricts the choice of usable detector configurations, thereby limiting the system's thermal sensitivity. In active scanning systems, the sinusoidal scan rate can cause undesirable variations in image exposure time. Secondly, non-tunable resonant scanners are operated to scan at the inherent resonant frequencies defined by the physically resonant elements of the scanners. Lastly, for applications wherein the scanning system must cover the largest possible field-of-view or scan angle, pupil shift must be reduced or eliminated to minimize scan mirror mass and associated torsion rod stress.
Resonant mechanical scanners which are tunable over a predetermined frequency range centered about the resonant frequency are now available for use in radiation scanning systems. The mechanical spring element of the tunable scanner may be fabricated from a material having a variable modulus of elasticity. The modulus may be varied by temperature using material such as 0.83% carbon steel or Nitinol, a nickel-titanium alloy, to form a thermally tunable scanner.
The modulus can also be varied by magnetic, electromagnetic or electrical energy using material appropriately reactive to the particular energy form. Alternatively, an electronically tunable magnetic spring may be coupled in series with the mechanical spring element of the scanner. In some cases it is desirable to phase lock the scanners by electrically driving them at a frequency slightly off of the mechanical resonance frequency.